Down Syndrome Screening Test

All parents wish to have healthy children. The health of your baby is assessed in many various ways during pregnancy. Blood tests and ultrasound scans are among the diagnostic methods used during pregnancy.

What is a Screening Test?

Tests performed on all individuals of a community to identify a particular disease in seemingly healthy individuals are called screening tests. Thanks to these tests, it is possible to detect and treat diseases early on. Some screening tests are mammograms (to detect breast cancer) and the PAP-smear test (to detect cervical cancer). However, a screening test is not a diagnostic test. Additional tests are required to confirm the diagnosis in patients whose screening tests yield positive results. Tissue sampling from the breast following the mammogram or from the cervix after the pap-smear test are examples of additional tests. These tests are called diagnostic tests.

Why aren’t diagnostic tests performed first?

Screening tests can be performed on all individuals, they are cheap, and they cause minimal discomfort (if any). Diagnostic tests are expensive procedures requiring biopsies, or special interventions such as amniocentesis, and laboratory analyses. As an example, an amniocentesis is required to the diagnose Down syndrome, amniocentesis (sampling from the fluid in which the baby is contained) should be performed. To perform this procedure, an experienced doctor and ultrasound scans are necessary. Additionally, there needs to be a geneticist to perform the chromosome analysis. The procedure carries a risk of causing miscarriage. Therefore, due to the risks and the cost involved, diagnostic tests are only performed on “high-risk” cases.

What is a Chromosome?

A chromosome is a DNA molecule which carries part or all of the genetic material of an organism, which makes us who we are, and which we pass onto our children. Humans have 23 pairs of chromosomes. The gender chromosomes are one of those pairs (XY for males; XX for females).

What is Down Syndrome (*)?

Down syndrome (trisomy 21) is a genetic disease caused by the presence of all or part of a third copy of chromosome 21. Children with Down syndrome have mild or severe mental retardation, learning disabilities and delayed physical development. Slanting eyes, shortness of the limbs and speech problems are the main physical features. They also have higher chances of cardiac anomalies, reflux, recurrent ear infections, sleep disorders and thyroid disorders. Approximately one out of 800-1000 babies are born with Down syndrome.

How is Down syndrome screened for during pregnancy?

In the 10-14 and 16-18 gestational weeks, a biochemical screening is performed through maternal blood sampling. Free beta-hCG and PAPP-A are analysed in the maternal blood at 10-14 weeks of gestation; and alpha fetoprotein, beta-hCG and oestriol levels are measured in the maternal blood at 16-18 weeks of gestation. These tests are not IQ tests. As you probably know, IQ cannot be measured through blood sampling.

First trimester screening

The obstetricians divide the pregnancy process into trimesters. The first 14 weeks are the first trimester. At 10-14 weeks of gestation, your doctor will determine your baby’s age, heart rate and nasal bone by ultrasound. At this time, the thickness of the neck oedema (NT) is assessed during these weeks. The baby’s age, NT measurement and blood tests (free beta-hCG and PAPP-A) are used to estimate the risk of Down syndrome.

Second trimester screening

In the second trimester, maternal blood is sampled at 16-18 weeks of gestation (to evaluate alpha fetoprotein, beta-hCG and oestriol). These tests are also used to estimate the risk of Down syndrome.

What is the Risk?

We talked about the logic of screening tests, but what do high-risk and low-risk mean?

The screening test is not a diagnostic test. The aim of screening tests is to perform the diagnostic test (chromosome analysis) and intervention (amniocentesis) as scarcely as possible after the screening test.

A “high-risk” or “positive” Down syndrome screening test does not mean your baby has Down syndrome. Likewise, a “low-risk” or “negative” result does not mean your baby does not have Down syndrome. At best, nine out of 10 cases of Down syndrome will be detected with this screening logic and methods, but one baby will be born with the syndrome. This figure applies to all centres in the world that implement a similar screening policy.

Then why not perform amniocentesis on everyone?

Amniocentesis is not a screening test, it is a diagnostic test. Perhaps, what is never told is that it is neither affordable nor healthy for the baby. Amniocentesis leads to miscarriage in 2-5 cases out of 1000. That is why it is only recommended for “high risk” cases.

Amniocentesis is performed on pregnant women who understand the risks of the procedure and discuss all possible consequences with their doctor.

Is screening possible with multiple pregnancies?

With multiple pregnancies, the blood values evaluated vary depending on the number of babies. That is why blood sampling is not used for screening purposes in multiple pregnancies. In multiple pregnancies, while NT and nasal bone measurements can be evaluated between 10-14 weeks of gestation, and ultrasonography findings related to Down syndrome in second trimester, it is important to keep in mind that ultrasound scans alone as screening methods are less reliable than when combined with blood analyses. Through the first trimester NT measurements, 7 out of 10 Down syndrome cases can be determined in multiple pregnancies. In multiple pregnancies, the diagnostic procedure is performed on all foetuses, not only on the foetus presenting a higher chance of Down syndrome.

Your job: To conduct your screening tests at the gestational weeks mentioned above and to discuss the test results with your doctor. Wishing you a healthy pregnancy.